I can't imagine Lion will require more then 2GB of RAM. Apple would be cutting off a huge chunk of their audience who bought Macbooks/Airs etc. (especially Airs which you can't add more RAM to). What's Snow Leopard require?
I suspect it's more a case of "what works" versus "what works well." Sure it'll be functional, perhaps even smooth, but put much load on it and you'll get frustrated. That was certainly my experience in working with a Mac Pro with 2GB of RAM in it. Threw in 4GB more and it was a totally different machine.